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E39 (1997 - 2003) The BMW 5-Series (E39 chassis) was introduced in the United States as a 1997 model year car and lasted until the 2004 when the E60 chassis was released. The United States saw several variations including the 525i, 528i, 530i and 540i. -- View the E39 Wiki

Personally, I'm leaning toward "a rotor is a rotor" ... as I have no anecdotal or personal experience otherwise ... but ... that's why I ask the question.

To be clear, I'm sure there 'can' be (perhaps huge) differences between rotors, mostly on the quality side ... but ... the question is whether ... in reality ... there is a meaningful difference between rotors of the most-recommended brands for our E39?

When you say "meaningful difference", I am assuming that you are referring to braking distance since that is the primary metric to measure braking performance. Since the brake rotor simply acts as a surface for the brake pad to grip onto to slow down a moving vehicle, I would agree that there should be no performance difference (e.g. stopping distance) between rotors that are made of the same material, assuming the same brake pads, equal quality of construction and street use. While most rotors are constructed of steel, I do not think the coefficient of friction varies significantly between the various grades of steel used. Any performance differences in fade, cooling, etc. would be due to design, mass, etc. Things like brake warping, out of round, etc. are quality issues outside the scope of this discussion. Other issues like brake squeal are probably more dependent upon the brake pad used, vice the rotor.

Probably correct...unless the rotors are manufactured with inferior raw materials or quality control isn't real a priority. The only difference I could discern was coated rotors don't corrode as quickly as non-coated. Other than that, as long the steel is of the highest quality obtainable and the rotors balance well, a rotor is probably a rotor.

The quality of steel and manufacturing process is why I wouldn't order a Chinese brand. I'd be worried about what happens the first time I go down a mountain and the brakes get hot. Ever see a Chery crash test video? They're probably out of round, unbalanced, and brittle.

When you say "meaningful difference", I am assuming that you are referring to braking distance

Good point!

I left 'meaningful' undefined because I wasn't sure what was meaningful.

I do agree, probably the 'most meaningful' metric is stopping feel & distance.

Quote:

Originally Posted by Fudman

Any performance differences in fade, cooling, etc. would be due to design, mass, etc.

Sounds reasonable that an OEM-shaped rotor would perform as OEM shaped-rotors do.

Quote:

Originally Posted by Fudman

Things like brake warping, out of round, etc. are quality issues outside the scope of this discussion.

I'm not so sure that's out of scope.

When I chose my first E39 replacement rotors, I simply researched the brands that you guys recommended - and simply selected the least expensive (taxed and shipped to my door) of those.

If it had been 'warped' (in quotes because what I think you meant was the faces were not parallel due to lousy manufacturing tolerances), or out of round (I'm not sure what that means), I would not have been happy.

So, I would disagree here and say that meeting the OEM specs would be as important a criteria as performance (i.e., stopping ability).

Quote:

Originally Posted by Fudman

Other issues like brake squeal are probably more dependent upon the brake pad used, vice the rotor

Agreed.

Quote:

Originally Posted by AnotherGeezer

coated rotors don't corrode as quickly as non-coated

I'd lump that under cosmetics and not 'meaningful' in a performance sense.

Quote:

Originally Posted by AnotherGeezer

the rotors balance well

Presumably you mean even distribution of weight.

Is weight imbalance (as opposed to size tolerance) a common problem?

How would we know if a rotor is not weight 'balanced'?

For example, following your recommendations, I noticed, in post 91 of the brake thread, that my new and old rotors had a weight-balancing slice taken out of the edge:
- What is this slice taken out of my rotor (1)

Note: Ignore the text on the picture - we found the answer (it was for weight distribution):

Quote:

Originally Posted by AnotherGeezer

a rotor is probably a rotor

Intuition says the more the marketing guys try to tell us otherwise, the more the case is that, yes, a (recommended brand) rotor of the OEM fitment will perform like any other (recommended brand) rotor of OEM fitment.

I believe that there is a definite difference between brake rotors. If the foundry does not have good technology and quality control, all sorts of inconsistent grain structure and inter-metallic compounds (hard spots) are probable. Besides, consider that many aftermarket brake manufacturers sell 2 or more grades of rotors with the price ratio approaching 2 to 1. While I doubt the cost ratio is that high, I'm pretty sure that there is a difference between the bottom & premium grades. Unless their product spec sheets and descriptions are pure fairy tales.

I think that many common brake vibration problems are a result of the irregularities throughout the rotor. Further, the temperatures created during braking can be high enough to cause changes in the grain microstructure and growth of inter-metallic compounds that will affect the surface condition of the rotor. Especially if the foundry practices are deficient. A lot of reading and a little personal experience leads me to believe that at the very least, many "warped" rotor are not in fact distorted, but rather have hard spots of intermetallics &/or resultant uneven brake pad deposits on the rotor friction surfaces due to the irregularities. Both will cause braking effect variation as the wheel rotates.

Have you ever had brake shudder/vibration without any pulsing in the pedal? Many of my brake problems over the years have been that way. Friction variation is the most likely explanation. Especially since I could make a good case that a pure warp, i.e. run-out, in a rotor should not pulse the pedal (particularly with single piston floating calipers), would create only insignificant braking variation and would cause only mild steering wheel shake. (Thicknes variation is another matter & would cause both pulsing pedal and brake vibration.)

A prior car (GM), bought brand new, went 100k miles on the original Delco pads & rotors without any vibration ever. No complaints there. I bought brand name premium grade pads and PG grade rotors. "PG" being premium grade, so I was told. After about 10k miles I had a vicious brake vibration, but no pedal pulsing. I had the rotors turned. OK for 10k miles, then vibration back. I checked into the rotor brand & grade. It turned out "PG" was professional grade. In other words, bottom line, enough to pass a safety check - just don't keep the car. A learning experience for me.
I checked run-out and thickness - all in spec. I then tried hand sanding the rotors with good abrasive paper to scuff up and remove any micro irregularities from the surfaces. Good for about 10k miles & then vibration returned. Eventually I put Delco pads & rotors on & problem gone for for another 100k until the car died.

For the extra hundred or so bucks on a brake job, I'm not going to take the chance on no name, unknown quality or low grade rotors. BMW, ATE, Zimmermann, etc. for me.

But I am willing to buy for as much off list price as I can find The adage that "you get what you pay for" needs very careful interpretation in real life.

I put Balo's on the front couple month ago. I don't know what to say, they are made in Germany and so far their performance is just what BMW does... Stops you like no other.

Also the pads are important factor in braking and stopping distance (IMO).
But from my experience few days ago when I went and bought lower arms from NAPA which they were the best they can make with life time warranty and they were more expensive than Lomforders, they just didn't fit on the car! The bushing side where there are a lot of teeth, didn't go in the housing where it is supposed to be bolted.

Stick to a German brand because they made the car and they know the sizes and every little spec on the car whereas other companies who try to make parts for those, they are just pathetic.

My point is that there will be small differences and you might not be able to detect them and small details/differences can be hard to detect on rotors but in my case (the control arms) it happened to be easy. They just didn't fit lol. The materials could be better don't get me wrong but if those engineers couldn't make it fit perfectly as it is supposed to, then personally I don't think they are capable of making something they claim to meet or EXCEED OE specs.... Maybe I shouldn't judge on a single experience but it was a very frustrating experience.

The quality of steel and manufacturing process is why I wouldn't order a Chinese brand. I'd be worried about what happens the first time I go down a mountain and the brakes get hot. Ever see a Chery crash test video? They're probably out of round, unbalanced, and brittle.

I'm the one who posted the Winhere thread. I bought those rotors because that's all I could get my hands on that day. If I'd had more time I might have waited for different rotors- Balo or maybe Zimmerman. But the Winhere rotors are working fine. I bedded in my Hawk pads on them pretty aggressively and not a trace of vibration or noise after 500 or so miles.

That being said, you never know what you're getting these days. I'd always bought nothing but Zimmerman rotors for my track 911. But after breaking two of them in a year, and hearing others tell me the same thing, I changed. I'd had good luck with Zimmermans on my track 911s for years but all of a sudden they've become less reliable. I also used nothing but SKF front wheel bearings for decades on the 911's but this past winter I couldn't get one to slip on the spindle. I pulled out my digital caliper and measured them and they were out of round. I had 6 sets in my parts bin and measured them all. 2 were bad- the first one that was out of round and a second one that had an ID that was smaller than all the others. The 4 good ones were made in Germany, the two crappy ones were from Indonesia. What I'd thought was a good German company was sourcing some of their parts from the far east and the quality control was junk.